1. Field of the Invention
The present invention relates to a method of controlling a component concentration of a plating solution in continuous electroplating which is preferably used for controlling the concentration of a plating solution in electroplating a metallic strip using an insoluble anode.
2. Description of the Related Art
A metallic strip is continuously electroplated by using a series of devices comprising a plating cell comprising an insoluble anode; a circulating tank for supplying a plating solution to the plating cell; a dissolution tank, connected to the circulating tank, for supplying to the circulating tank the plating solution in which plating ions are adjusted; a metal feeding device and an acid feeding device for feeding a metal and an acid, respectively, to the dissolution tank; and a plating system.
In this case, the concentrations of plating solution components such as metal ions, sulfuric acid, etc., which are dissolved in the plating solution, are controlled. In controlling the concentrations, in order to maintain the concentration of metal ions at a predetermined target value, the sum of an estimated metal consumption (feed forward control) computed by estimating the consumption by plating under plating conditions, and a deviation between a predetermined concentration target value and an actual concentration value is set as a feed of a metal (feedback control).
In regard to such a plating technology, for example, Japanese Unexamined Patent Publication No. 2-217499 discloses as a method of controlling a concentration of the plating solution for alloy electroplating a technology in which the plating electricity supplied is measured, and the consumption of metal ions in a plating bath is computed based on the measurement of electricity so that at least one of metal ions, free acid and water is adjusted.
Further, Japanese Unexamined Patent Publication No. 5-320997 discloses as a method of controlling a concentration of metal ions in a zinc alloy electroplating solution a technology in which the amount of a metallic salt to be supplied is determined from the sum of a reference feed of the metallic salt computed on the basis of a plating current and a dragout amount of the plating solution (amount of the plating solution which flows to the outside of the plating system), and a corrected amount of the metallic salt supplied which is computed on the basis of the component concentrations and the pH value of the electroplating solution so that the concentration of the metal ions in the electroplating solution is controlled by supplying the determined amount of the metallic salt to the electroplating solution.
In both methods of controlling a concentration in electroplating disclosed in the above publications, the target value of feedback control is set to a value fixed as a predetermined target value.
However, in continuously electroplating a metallic strip, the plated metallic strip is washed with water at the outlet of the plating cell, and the washing water and the plating solution which adheres to the metallic strip are recovered to the circulating tank. Therefore, the whole washing liquid is diluted with the water which flows in from the outside of the system, and thus excess water is evaporated to the outside by an evaporator unit to balance the washing liquid.
Even in such a case in which electroplating is carried out while balancing the inflow of water in the system and the amount of evaporation to the outside of the system, one of both amounts becomes excessive and thus causes unbalance therebetween, thereby creating a variation in the total amount of the plating solution present in the system comprising the above-described series of devices. In this case, the application of the feedback control method comprising setting the control target value to a constant and canceling a difference between the target value and the actual value of the concentration of metal ions, as disclosed in the above publications, causes the following phenomenon.
For example, when the inflow of water is larger than the evaporation of water, as shown in FIG. 4A, since the total amount of the plating solution in the system is increased to dilute the plating solution, a metal is fed by feedback control in order to supply the metal in an amount corresponding to a decrease in the metal ions between times t.sub.1 and t.sub.2. Furthermore, in order to maintain the metal ions concentration of the metal at a constant target value, as shown in FIG. 4B, and to maintain the concentration of sulfuric acid, which is decreased due to the consumption by dissolution reaction between the fed metal and sulfuric acid, sulfuric acid is supplied.
Although the total molar concentration is used as the concentration of metal ions here, for example, when the plating solution contains at least two types of metal ions, as an alloy plating solution containing zinc and nickel, the total molar concentration represents the total of the concentrations of these metal ions.
In the above-mentioned plating, if the washing water of the strip is discharged to the outside of the system, the metal and acid need not be supplied. However, since the plating solution contained in the washing water is disposed of, the unit consumption of the metal and acid is diminished. When the washing water is discharged after treatment, and water is discharged to the outside of the system by evaporation, the amount of the plating solution is gradually decreased, thereby increasing the concentration of metal ions. Hence, the above method of balancing the amount of the water which flows in the system and the amount of water evaporated becomes effective.
However, when the control technologies disclosed in Japanese Unexamined Patent Publication Nos. 2-217499 and 5-320997 are applied to the electroplating system comprising flowing in and out water, since the target value of feedback control is set to a constant target value, unbalance between the inflow of water from the outside and the evaporation of water to the outside causes the several problems.
The problems will be described with reference to FIG. 4. If the inflow of water from the outside is larger than the evaporation of water to the outside between times t.sub.1 and t.sub.2, thus diluting the plating solution, as described above, excess metal is supplied for canceling the dilution in order to keep the concentration of metal ions constant by feedback control. As a result, the excess metal supplied is dissolved in sulfuric acid by reaction formula (1) below, and thus sulfuric acid is also consumed and reduced in amount. EQU Zn+H.sub.2 SO.sub.4 .fwdarw.ZnSO.sub.4 +H.sub.2 .uparw. (1)
When the amount of sulfuric acid is reduced, as described above, sulfuric acid is supplied in order to maintain the sulfuric acid concentration of the plating solution at a target concentration by feedback control. As a result, at time t.sub.2 the total amount of the plating solution is at a maximum, but both the concentration of metal ions (total molar concentration) and the sulfuric acid concentration remain at the target values, despite the significant increase in the amount of the plating solution.
Since water is then discharged by evaporation between times t.sub.2 and t.sub.3, and plating is effected in order to maintain the concentration of the metal ions at the target value, at time t.sub.3 the total amount of the plating solution returns to the target value, but there is no escape of excess sulfuric acid supplied for dissolving the metal. This makes it impossible to return the sulfuric acid concentration of the plating solution to the target value, as shown in FIG. 4C, and thus causes a decrease in pH of the plating solution with an increase in the concentration of sulfuric acid.
This phenomenon will be described in further detail below with reference to the case of a zinc (Zn) metal as an example. In electroplating, since plating reaction (electrodeposition reaction) represented by the following reaction formulae (2) and (3) proceeds, the amount of sulfuric acid is balanced by an increase due to plating reaction represented by reaction formula (3) and a decrease due to reaction with the supplied metal represented by reaction formula (1). EQU Zn.sup.2+ +2e.sup.- .fwdarw.Zn .dwnarw. (2) EQU H.sub.2 O+SO.sub.4.sup.2- .fwdarw.H.sub.2 SO.sub.4 +1/2O.sub.2 .uparw.+2e.sup.- ( 3)
FIG. 2 shows the conception of this relation with reference to pure zinc (Zn) plating as an example. The concentration of metal ions is shown on the ordinate, and the sulfuric acid concentration is shown on the abscissa. Both concentrations are controlled to be balanced at the intersection of both target values.
Namely, as is obvious from the above formula (2), since the excess metal supplied can be removed by consumption in plating, the concentration of metal ions can be controlled to the target value by performing plating at the same time as evaporation of water, as shown in FIG. 4B. However, in this case, at the same time, the amount of sulfuric acid is increased due to reaction represented by formula (3). Since there is no escape of the increased amount of sulfuric acid, as described above, the concentration of sulfuric acid in the plating solution is increased, i.e., the pH thereof is decreased, thereby causing various problems.
The same phenomenon is produced in alloy plating. Although contents will be partly duplicated, this alloy plating will be described below.
In the electroplating system for continuously plating a metallic strip, generally, water used for washing the surface of the plated metallic strip is recovered in the plating system, as described above. Since the plating solution is diluted with the water which flows in the system, excess water is removed by evaporation to the outside of the system by using an evaporator unit. However, the amount of evaporation cannot be continuously changed by the evaporator unit, and thus the actual value of the total amount of the plating solution (the total of the plating solution present in the plating system) inevitably varies by about 5 to 10% relative to the target value.
When the actual total amount of the plating solution is increased due to inflow of water from the outside of the system, as described above, the measurement of the concentration of metal ions (g/l! or mol/l!) is consequently decreased. Therefore, in this case, in order to keep the concentration of metal ions constant, it is necessary to counteract the decrease in metal ion concentration by feeding a metallic agent (for example, a metal itself, a salt thereof, an oxide thereof, or the like) containing the object metal into the plating solution.
This will be described with reference to a sulfuric acid plating solution for alloy plating as an example. In this case, it is necessary to feed Zn and Ni agents (metallic agents). In feeding such agents, when the Zn agent is ZnO, and the Ni agent is metallic Ni, the amount of sulfuric acid is decreased, i.e., the pH is increased, according to the reactions represented by the formulae (4) and (5). Thus, in order to maintain the concentration of sulfuric acid (or pH) at a constant value, sulfuric acid must be fed. EQU ZnO+H.sub.2 SO.sub.4 .fwdarw.ZnSO.sub.4 +H.sub.2 O (4) EQU Ni+H.sub.2 SO.sub.4 .fwdarw.NiSO.sub.4 +H.sub.2 .uparw. (5)
In electroplating Zn--Ni alloy, in general, metal (Zn, Ni) ions are consumed by electrodeposition reaction represented by the same formulae (6) and (7) below as the above formula (2) on the surface of a cathode (metallic strip), and at the same time, the amount of sulfuric acid is increased due to reaction represented by the same formula (8) below as the above formula (3) on the surface of an anode. In this alloy plating, therefore, the consumption of metal ions by plating reaction of formulae (6) and (7) is made up for by the reactions represented by the above formulae (4) and (5) so as to balance the concentration of metal ions and the concentration of sulfuric acid (or Ph). EQU Zn.sup.2+ +2e.sup.- .fwdarw.Zn.dwnarw. (6) EQU Ni.sup.2+ +2e.sup.- .fwdarw.Ni.dwnarw. (7) EQU H.sub.2 O+SO.sub.4.sup.2- .fwdarw.H.sub.2 SO.sub.4 +1/2O.sub.2 .uparw.+2e.sup.- ( 8)
Therefore, when the total amount of the plating solution is increased by the inflow of water from the outside of the system, feeding of the metallic agents and sulfuric acid in order to keep the component concentrations constant makes excessive both the metal ions and sulfuric acid at the time the total amount of the plating solution is then returned to the target value by evaporating water. Thus, the feeding of the metals and sulfuric acid must be stopped. In this case, since not only the supply of metal ions according to the above formulae (4) and (5) is stopped, but the metal ions are consumed by continuing plating according to the formulae (6) and (7), the amount of metal ions can steadily be reduced. However, since the amount of sulfuric acid is not reduced according to formulae (4) and (5), but it is increased according to the above formula (8) by continuing plating for returning the concentration of metal ions to an appropriate value, similarly, the concentration of sulfuric acid is further increased, and the pH is decreased.